Production of sulphur dioxide from metal sulphates



May 28, 1940.

B. .BARNES ET Al. l 2,202,414. PRODUCTION 0F SULPHUR DIOXIDE FROM METAL SULPHATES Filed Dec. 3, 1938- 3 Sheets-Sheet 1 ATTORNEY.

24k-fw,

My 28,. 1940. a. BARNES ET Al.

PRODUCTION 0F SULPHUR DIOXIDE FROM METAL SULPHATES Filed Dec. 5, 1958 3 sheets-sheet 2 ATTORNEY.

May 28, 1940- B. BARNES Erm. 2,202,414

` PRODUCTION 0F SULPHUR DIOXIDE FROM METAL SULPI-IATESA Filed Dec. :'s, 1938 s sheets-sheet s NDW INV ENT ORS ATTORNEY.

Si I rs Patented May 28, 194() lPATENT lolulcla;

-. PRODUCTION F SULPHUR. DIOXIDE FROM v METAL -SULPHATESK l Blakeslee Barnes, New York, N. Y., and Harold E. Broughton, 'Hackensack N. J., assignors to Chemical Construction'. Corporation, New York, N. Y., a corporationof Delaware Application December 3, 193s, serial No. 243,706

' w 14`c'1aims. '(01.23-177) UNITED STATES This invention` relates, to ther production of sulphur-'dioxider gases from heavy metal sulphates such as iron sulphate, copper sulphate, zinc sulphate, and the like. More -particularly,the invention relates to amethod of obtainingirom heavy metal sulphates of, this class a sulphur dioxide gas of suiiicient, strength and xfree oxygen content for catalytic oxidation by the contact sulphuric'acid process. m The roasting of. heavy-metal sulphates for metallurgical purposes .has been practiced for many years, and the sulphur dioxide gases obtainedl iront-this source have frequentlybeen converted to sulphuric acid by the .chamber process since this process willv operate on relatively Weak gases.. Such ores as iron pyrites, .zinc sulphate and the like have. also been roasted'in 'Herrsho furnaces and similar heavy equipmentv under conditions such thata 6-7% sulphur o ydioxide gas has been4 obtained.'` Heavy metal sulphates of vthis class have also -been mixed with fuel if necessary and sintered Yon the Dwight- Lloyd sintering machine, but `heretoforethe sulphur dioxide gases recovered by this process have been too Weak forf catalytic conversion. I

We have now discovered that heavy metal sulphates, and particularly. hydrated `ferrous sulphates such as are vobtained as Icy-products yfrom the sulphuric acid leachingfof ilmenite in lpig- O ment manufacture and from the pickling of iron mixture With carbonaceous `fuel in such a mannerthat a sulphur dioxide `gas suitableV for cataupon the closecontrol of a number of factors as Y will subsequently be described.

It is therefore a prmcipal obJect of the presfgvve avery dilute gals, but Wehave found that .40

ent invention to provide a method of roasting and/or sintering heavy metal sulphates in admixture with carbonaceous fuel in suchv atmam ner as to obtain a strong sulphur dioxide gas I suitable for catalytic conversion: Another object of the invention is to provide av granulated charge of a mixture of finely. divided heavy metal sul-A phate and carbonaceous-,fuel which can/'be ignited and sintered or roastedin a 'current of air with rapid evolution of a'stro'ng sulphur dioxide ,gas and the 'productionrof a hard, ldense 'metal-oxide sinter of low sulphur content. lv Furtherobjects f include the'control of moisturecontent, amounts of fuel, size of granulation, and'other .variables in lorder to obtain the optimumv results. Still and steel, can be roasted and/ory sintered `in ad- Y further `objects Will be apparent from the following description, when taken with the drawings attached hereto, and will be pointed' out in the appended claims.V While the broaderzprinciplesof the invention 5 may be applied to any heavymetal sulphatecapable of reduction by a carbonaceous fuel, we believe that its most important commercial advantages are obtained in the treatment of ferrous sulphate. be described in detail with particular reference to ferrous sulphate monohydrate o-btained fromv spent pickle liquors or from the manufacture of titanium pigments, :but it should be'understood that this description illustrates the broad prin- 15 ciples of the inventionjas Well;r Theinvention in its broader aspects is limited-only by the 'scope' of the claims annexed hereto.V kOn Vthe attached drawings, -Figgl illustrates diagrammatically a process for the concentration o and recovery of hydrated ferrous sulphate from spent pickle liquor or from titanium oxide manufacture and thel production of a strong sulphur dioxide gas therefrom. Fig. 2 is a graph showing the compositions of gases obtained when roastg5 ingy ferrousy sulphate from this source. Fig. 3

is a similar graph 'showing the results obtained when a mixture of carbon and pyrite is used as fuel.

rapidly. In order to obtain such rapid combustion, We have found Athat the sulphates mustv be mixed with carbonaceousfuel and granulated A'85 and then burned in a relatively thin, porous layer While passing a strong r current of air through them. The completedesulphurization of the vmaterial by such an Iair blastu'wouldyof course,

more than of `the sulphur is evolved las sul,- phur dioxide in the first portions of the products vof combustion, so/thatv a strong gas is .obtained `4When these products remainder. i

are separated from` the j Accordingly, the invention Will 10 Our inventionl is basedon the discovery that =30 .a strong sulphur dioxide gas is obtainable"` from heavy metal sulphates only when theyare burnedl per sq. ft. of surface area produces the best results.

The amount of fuel employed also has an important influence on the results obtained. Too much carbonaceous fuel results in too high a combustion temperature, which gives a sinter high in sulphur and a Igas having an excess of carbon dioxide and a deficiency of free oxygen,

while too little fuel produces a very dilute gas and a sinter of low mechanical strength. An'4 amount of fuel roughly equivalent to 6-12% of coal on a heat evolution basis produces the best results with most heavy metal sulphates, and a part of this fuel charge may be made up of sulphur-containing material such as a free sulphur, pyrite, o-r other heavy metal sulphides, thereby increasing the strength of the resulting SO2 gases. However, it is necessary in all cases to have at least 3% of carbon present based on the total weight of 'the charge, since sulphur alone will not produce a. gas free of sulphur trioxide.

The further details of the process as applied to hydrated ferrous sulphate will be discussed in conjunction with the equipment illustrated in Fig. 1. Referringto this gure, a concentrator for waste ferrous sulphate liquors is shown at I, this concentrator being of the well-known drum type operating with hot products of combustion from an oil burning furnace 2. Waste ferrous sulphate liquors are introduced at 3 and passed in counter-current with hot gases passing through the pipes 4 and 5 and the concentrated liquor leaves through outlet pipe 6. This liquor may be cooled and mixed with strong sulphuric acid in a cooling and crystallizing vessel 'I and crystals of ferrous sulphate heptahydrate removed therefrom in a rotary lter of the Oliver type such as the filter 8, after which the filtrate is preferably returned to the pickling plant for reuse.

The heptahydrate crystals are introduced into a rotary dehydrating kiln 9 where the greater part of their water of crystallization is removed by counter-current contact with hot gases from an oil or gas fired furnace Il). Instead of introducing heptahydrate crystals into this dehydrator, the concentrated liquor from the concentrator I may be dehydrated directly, as indicated by the dottedv flow line I I.

It should be understood that the invention in its broader aspects is not limited to the above .method for obtaining hydrated ferrous sulphate and similar heavy met'al sulphate crystals. On the contrary, there are a number of other methods available for accomplishing this purpose; for example, waste ferrous sulphate liquors may vbe mixed with strong sulphuric acid, concentrated and lteredat temperatures above 65 C. whereby ferrous sulphate monohydrate is obtained, as described in the Smith and Belding Patent No. 2,185,095 dated December 26, 1939. Alternatively, instead of concentrating the waste liquors, they may be subjected to artificial refrigeration with or without the preliminary addition of sulphuric acid, whereby crystals of ferrous sulphate heptahydrate are obtained. 'Ihe broad principles of the'invention are directed to the production of a strong sulphur dioxide gas from solid heavy metal sulphate crystals, and these crystals may be obtained from any source and by any suit'able method.

The ferrous sulphate leaving the dehydrator 9 may have a Water content of about 14-20% and is preferably in the form of ferrous sulphate monohydrate containing about 16% of water. These crystals are finely divided, and are preferably of a size sufficiently small to pass through a standard 6 mesh sieve. The nely divided particles are fed to a rotary granulator or pelleting mill I2, which is preferably of the type known as a pug mill and consists of a rotary shell equipped with internal paddles moving in a direction opposite to its direction of rotation. This mill is charged with the hydrated ferrous sulphate described above together with a metal oxide such as iron oxide, an amount of carbonaceous fuel sufficient' to produce a gas of optimum strength for the subsequent roasting operation, and a binder such as waste sulphite liquor, crude molasses residues, or water.

Pellets or granules of very uniform size are obtained from the pug mill granulator and these are introduced into a drier I3 for suitable reduction in their moisture content. This drier is preferably of the rotary type in which the granules pass downwardly in counter-current contact 'with drying gases ofrelatively high temperature. Waste gases of. low sulphur dioxide content from the subsequent sintering step are well suited for use as the vheating and drying medium as they are available at temperatures of '70S-800 F., or hot gases from other sources may be used. For best results, the pellets or granules should preferably be dried t'o a final temperature of YZO-180 F. and a-total moisture content of not more than 12%.

The dried and hardened pellets are then 'classified on a vibrating screen of 1A" mesh size, the oversize material being crushed and returned to the granulator for reworking. When proper vworking conditions are maintained in the granulator and drier it will be found that not more 'than 10-15\% of oversize material is obtained and care should -be taken that not more than 10% of nes are mixed with the pellets passing through the screen. l

The dried and sized pellets are retained in a storage hopper I5 and are supplied as needed to the feeding device I6 of a roastingand/or sinterving machine of the moving grate type, such as a Dwight-Lloyd machine, a Greenvvalt machine, or the like. A machine of the Dwight-Lloyd type is yillustrated at Il and operates as follows:

"The machine is essentially a steel framework supporting a closed track around which travel a yseries of movable grates or pallets 2U for carry- -ing the charge, together with a driving mechanism, an igniter I8, a feed hopper I9 and a plurality of suction" or wind boxes 2I, 22, 23, and

24 directly beneath the upper pallet track section. Exhaust fans are connected with these wind 'boxes for drawing air through the charge on the pallets 20 and for carrying away the products of combustion.

The operation of the machine is continuous. As each pallet passes under the feed hopper it receives a charge of combustible material and carries it under the igniter I8 where a flame is directed upon the topsurface to start the charge burning. The pallet then passes over the rst wind boxes, 12|, 22 which draw a blast of air through the charge and receive the first portions of the resulting products of combustion. Continuing its travel itthen passes over the remaining vboxes 23 and 24 which draw further amounts of air through the charge and carry off the remainingfproducts of combustion, and its speed of travelis so .adjusted that thefpallet leaves .the last suction box zone at the moment parts of finely divided carbonv are added, depend- 'pilot' flame 8 which is sufficiently intenseto lightv off the charge in about one-half minute.

when the charge lhas been completelyfsinte'red.

The pallet thenpasses around the curved track section and discharges the sinter cake into a trough 25, from which a portion of itis Withdrawn for sale as at 26. The remainder falls upon a conveyor 21 which carries it to the hopper 29 of an elevator 30, the outlet 3l of which is adjacent thev return rolls 33 which discharge into the feed inlet 32 of the granulator I2. I-Ierey the sinter cake is crushed to 6 mesh size and mixed with ferrous sulphate, carbcnaceous fuel and a binder to form a new charge for granulation.

For purposes of the present invention, the co-nstruction of the above described sintering machine is modified by providing a means for collecting as a separate fraction the products ofcombustion evolved near the feed end of the moving grates by the first wind'boxes 2l and 22. As

is shown in Fig. 2, the combustion or sintering of a highly porous mass of granulated or pelletized ferrous sulphate will produce during` the.

first few minutes a strong sulphur dioxide gas containing the major part of the sulphur, after which the sulphur dioxide content of the gas soon f falls to zero. By providinga separate gas takeoff and suction fan for the first two Wind boxes 50-60 parts of crystalline ferrous sulphate obtained as a by-product from the treatment of ilmenite or other iron-titanium oresv and having a total Water content of lei-20% are ground if necessary to 6 mesh size and mixed with 25-40 parts of iron oxide, which may be partly in the form of fine ue dust and partly sintered FesO4 crushed to'6 mesh particle size. From A6 to 12 ing on the water content of the ferrous sulphate crystals, the larger" amounts being used when more Water is present. The mixture is thoroughly blended and granulated yin the pug mill granulator vI2 with'the vaddition. of about 8 parts -such that the pellets leave the kiln with a temperature of 1'70-180o F. and a total water content of not over 12%, and preferably about 8-10%. The pellets are then passed over a double vibrating screen, having upper openings of 1/4 inch in size and lower openings of about l0 per inch. The material remaining on the 'upper screen andV that passing through the lower screen is preferably passed through crushing rolls and then returned tothe granulator I2.

The pellets remaining on the lowerscreen'are fed onto 'the moving grate'of `the sintering ma-` chine Il in a relatively thin, porous layer about 6 to 7 inches deep. They are then .ignited bythe tion of 4 to '7 inches of water is maintained on `the wind boxesby the exhaust fans, and the current orblast of air so produced is sufficient to 'burn out more than 99% of the sulphur in the first eight to nine minutes of combustion. Complete desulphurization and sintering are obtained in about1520 minutes, depending on the amount of Water in the charge and the amount of fuel used,` and a sintered magnetite (Fe304) suitable for blast furnace 'reduction is produced.

The composition of the gases issuing from the wind boxes 2ll and 22, and representing thefirst portionsof the products of combustion is illustrated on Fig.- 2 ofthe drawings. These gases will average about '7% SO2 by volume with a sulphur recovery of 94-95'% and an oxygen content of about 10% by volume.l .Their temperature is about 780-800" F., 'and they are therefore well suited for direct introduction into a contact sulphuric acid plant provided with a vanadium catalyst.

Ironsulphateobtained by the concentration and dehydration of Waste ferrous sulphate liquors from .titanium-iy pigment manufacture has about the following composition:

A suc- Percent FeSO4 40.()l F62(SO4)3 31.9

Free H2SO4 5.7 Total water 16.3

Balance (probably Fe203) 6.1

A charge Was prepared containing 55% of this material, 29.5% of sintered magnetite in the form of crushed returns from the sintering machine, v'7.5%.of coal and 7.5% of pyrite, all of a particle size small enough to pass through a 6 mesh sieve. 0.5% ofl Walter was added and the mixture Was granulated, driedy and hardened, screened and fed to the sinteringk machine I1, as described in Example 1. The sulphur dioxide content of the gases evolved from this mixture is shown'on Fig. 3 of the drawings, from which it is apparent that a strong gas suitable for catalytic oxidation by the contact sulphurc acid process is obtained by collecting as a separate fraction the gases evolvedduring the first 5-6mir`1utes of combustion. After about 18 minutes ytotal combustion time at gas temperatures of S30-840 F., a good `hard Aand. dense sinter `was obtained having a Example 3 Spent pickle liquor from the pickling of sheet metal and containing -approximately 8% sulphuric acid and '20% FeSO4 is heated in the concentrator. I with hot gases, which may be the products of combustion from the furnace 2 or may be; the hot gases from the last wind boxes 23 and 24 of the sintering machine, or both.

41% of the Water content of the liquor is evaporated after which the remainder is mixed with about 13%l 4of 'its weight of concentrated sulphuricacid, coo1ed,and the resulting wet crystals of hydrated yferrous sulphate are filtered off by lthe filter 8. These crystals are further dehy- ,vdratedito aptotal Water content of aboutA .16% 4in lco the following composition is obtained.

' Percent FeSO4 58.8 Fe2(SO4)3 25.0 Water 16.2

A mixture of 30 parts of this material, which corresponds to a ferrous sulphate monohydrate containing some ferric sulphate, together with 20.2 parts of iron oxide returns from the sintering machine and 3.3 parts of coal, all in nely divided condition, were granulated in the granulator I2 with the addition of 2.8 parts of Water. The granules were dried and hardened by heating in a current of hot gases to a final temperature of V70-180 F. and a Water content of 11%, after which they were fed to the grate of a sintering machine, ignited and burned. Samples collected from the products of combustion gave the following results:

Upon completion of the combustion, 28 parts of a good, hard iron oxide sinter were obtained which contained less than 0.1% of sulphur and Were therefore Well suited for use in the manufacture of iron and steel.

What We claim is:

1. A method of obtaining from heavy metal sulphates a sulphur dioxide gas of sufficient strength and free oxygen content for catalytic oxidation by the Contact sulphuric acid process which comprises mixing said sulphates in the form of finely divided solids with heavy metal oxides and finely divided carbonaceous fuel in the presence of a binder, granulating and sizing the mixture to a fairly uniform size, drying and hardening the granules, igniting and burning the dried granules in a relatively thin, porous layer in about 20 minutes or less While passing a current of air through them, and collecting as a separate fraction the first portions of the resulting products of combustion.

2. A method of obtaining from heavy metal sulphates containing Water of crystallization a sulphur dioxide gas of suiiicient strength and free oxygen content for catalytic oxidation by the contact sulphuric acid process which comprises mixing said sulphates in the form of nely vdivided solids with an oxide of the corresponding metal and finely divided carbonaceous fuel in the presence of sufficient free Water to act as a binder, granulating the mixture, drying the granules to a Water content of not more than 12%, sizing the granules to a fairly uniform size, igniting and burning the dried granules in about 20 minutes or less in a relatively thin, porous layer While passing a current of air through them, and collecting as a separate fraction the first portions of the resulting products of combustion.

3. A method of producing a sulphur dioxide gas suitable for catalytic oxidation by the contact sulphuric acid process which comprises mixing together a finely divided heavy metal sulphate, a finely divided mixture of carbon and a combustible heavy metal sulphide and a heavy metal oxide in the presence of a binder, granulating and sizing the mixture to a fairly uniform size,

l the dehydrating kiln 9, whereupon a material of drying 'and hardening the granules, igniting and burning the granules in about 20 minutes or less in a relatively thin, porous layer While passing a current of air through them, and collecting as a separate fraction the first portions of the resulting products of combustion.

4. A method of obtaining from hydrated ferrous sulphate a sulphur dioxide gas of sufficient strength and oxygen content for catalytic oxidation by the contact sulphuric acid process which comprises mixing it in the form of a finely divided solid material with iron oxide and with finely divided carbonaceous fuel in the presence of a binder, granulating and sizing the mixture to a fairly uniform size, drying and hardening the granules and igniting and burning them in about 20 minutes or less in a relatively thin, porous layer While passing a current of air through them, and collecting as a separate fraction the first portions of the resulting products of combustion.

5. A method of obtaining from hydrated ferrous sulphate a sulphur dioxide gas of sufficient strength and oxygen content for catalytic conversion by the contact sulphuric acid process which comprises mixing it in the form of a nely divided solid material with iron oxide and with finely divided carbonaceous fuel in the presence of a binder, granulating and sizing the mixture to a fairly uniform size, drying and hardening the granules, feeding the granules onto a moving grate in a relatively thin, porous layer and burning them in about 20 minutes or less While passing a current of air therethrough, and collecting as a separate fraction the products of combustion evolved near the feed end of said grate.

6. A method according to claim 5 in which the granules contain an amount of fuel equivalent, on a heat evolution basis, to 6-12% of coal, and in which at least 3% is carbon.

7. A method of obtaining from hydrated ferrous sulphate a sulphur dioxide gas of suiiicient Strength and oxygen content for catalytic conversion by the Contact sulphuric' acid process Which comprises mixing it in the form of solid particles of 6 mesh or less in size With carbonaceous fuel of about the same particle size and With iron oxide in the presence of suflicient free Water to act as a binder, granulating the mixture to a 1A particle size, drying and hardening the granules, screeningr and returning the oversize material for grinding and regranulation, feeding the sized granules onto a moving grate in a relatively thin, porous layer and burning them in about 20 minutes or less while passing a current of air therethrough, and collecting as a separate fraction the products of combustion evolved near the feed end of said grate.

8. A method of producing a sulphur dioxide gas suitable for catalytic oxidation by the contact sulphuric acid process which comprises mixing hydrated ferrous sulphate crystals with iron oxide, pyrite and carbon, all of less than 6 mesh in particle size, granulating and sizing the mixture to a fairly uniform size in the presence of a binder, drying and hardening the granules, feeding the granules onto a moving grate in a relatively thin, porous layer and burning them in about 20 minutes or less While passing a current of air therethrough, and collecting as a separate fraction the products of combustion evolved near the feed end of said grate.

9. A method according to claim 8 in which the granules are dried to a total Water content of not over 12%.

10. A method of producing a sulphur dioxide gas suitable for catalytic oxidation by the contact sulphuric acid process which comprises mixing finely divided ferrous sulphate monohydrate With finely divided carbonaceous fuel and iron oxide, granulating the mixture in the presence of a binder, drying and hardening the granules. sizing the granules to' a fairly uniform size, igniting and burning the dried granules in about 20 minutes or less in a relatively thin, porous layer while passing a current of air through them, and collecting as a separate fraction the first portions of the resulting products of combustion.

11. A method according to claim 10 in which the granules are dried to a final temperature of 17o-180 F. and a total moisture content of not more than 12%. v

12. A method of producing a sulphur dioxide gas suitable for catalytic oxidation by the Contact sulphuric acid process which comprises preparing a charge containing about 50-60 parts of finely divided ferrous sulphate monohydrate, about -40 parts of finely divided iron oxide, about 6-12 parts of finely divided carbon and about 8 parts ofwater, granulating and sizing the charge, drying and hardening the granules to a fairly uniform size, igniting and burning the granules in about 20 minutes or less in a relatively thin, porous layer While passing a current of air through them, and collecting as a separate fraction the first portions of the resulting products of combustion.

13. A method of recovering from Waste ferrous sulphate liquors a sintered iron oxide and a'. sulphur dioxide gas of sunicient strength andl f free oxygen-content for catalytic oxidation by the contact sulphuric acid process which comprises concentrating said liquors and recoveringthe ferrous sulphate therefrom in the form of finely divided monohydrate, 'mixing iron oxide, finely divided carbonaceous fuel and Water with said ferrous sulphate monohydrate, granulating vand grate.

14. A method of recovering from waste ferrous 5 sulphate liquors a sintered iron oxide and al sulphur dioxide gasof suicient strength and free oxygen content for catalytic oxidation by the contact sulphuric acid process which comprises concentrating said liquors and recovering the ferrous sulphate therefrom in the form of finely divided monohydrate, mixing iron oxide, finely divided carbonaceous fuel and Water with said ferrous sulphate monohydrate, granulating the mixture to pellets ofv about IAL" size, drying said pellets by counter-,current contact with hot gases to a total moisture content of not more than 12%, feeding the dried granules onto the moving grate of a sintering machine in a relatively thin, porouslayer, igniting and burning the granules to a sintered mass substantially free from sulphur while passing a current of air therethrough, co1- lecting as a strong sulphur dioxide gas the products of combustion evolved near the` feed end of said grate, and returning the remaining products of combustion to the-'drying step.

BLAKESLEE BARNES. HAROLD E. BROUGHTON. 

